Often times fluid regulators, especially “boot style” regulators where the diaphragm (“boot”) acts as both the actuator/sensing member and as the sealing member, can exhibit instability at low fluid flow conditions. Because the diaphragm is used to engage and seal and move away from a sealing surface (e.g., of a cage) to prevent/allow the flow of fluid, during low flow conditions the travel is so small that any minor fluctuation in the fluid flow results in the regulator shutting off and then overcorrecting by opening too much. The regulator then cycles between closing and opening and often times the frequency of these fluctuations can be fractions of a second.
Attempts to reduce instability in these regulators usually involve using stiffer springs, lower gain, smaller control lines, or adding friction. However, these attempts have had limited success in reducing instability and often require several return trips, trial and error, and parts kits.